It is important to determine the eye position and viewing direction for very different applications, for example in the field of medical engineering. In the case of laser-based eye surgery or laser ablation, in which corneal tissue of the eye is ablated by means of laser pulses, the precise eye position must be known. However, establishing the eye position is also essential in the field of balance research, as it is in the further development of human/machine interfaces, for example in the field of vehicle control. In the field of advertisement and visual communication, knowledge of the viewing direction of the observer is an important factor for assessing the quality of graphics, for example in view of incentives for purchasing, which are offered on websites, billboards and videos.
In order to measure and track the eye position, use is made of so-called “eye trackers”. In the case of a laser treatment, this is used to determine the correct alignment of a laser beam in order to ensure the precise position of the tissue ablation. This is necessary because rapid eye movement may occur during a treatment and this can lead to a positioning error of the laser pulses relative to the eye. The so-called pupil tracking supplies the lateral position of the pupil (x-direction, y-direction), while a cyclotorsion tracker determines the rotation of the eye about the viewing direction (z-direction).
Document DE 102004020356 A1 describes a method for capturing eye movements, in which image sequences of an eye are captured using an optoelectronic sensor. Here, an area of the head is provided with markers, which are likewise captured. The movement of the eye in the head is established from the obtained image data, taking into account the pupil center and the markers on the head.
Patent DE19954047B4 shows a method for measuring eye movements, in which non-toxic tincture markers are applied with a predetermined geometry onto the conjunctiva of the eye and image sequences of the eye are captured using an optoelectronic sensor. From this, coordinates of a rotationally neutral reference point are established which coordinates are related to the coordinates of the markers in order to calculate the eye position therefrom.
Document DE102008049846A1 shows a further method for measuring the eye movement. Here, the sclera of the eye is illuminated by coherent light such that a spot pattern is created by interfering light which was scattered on the sclera. A displacement of the recorded spot pattern is established on the basis of two cropped images, and the eye movement is determined therefrom.
Document DE102008039838A1 describes a method for applications in medical engineering for scanning the three-dimensional surface of an object by means of a light-beam scanner, in which the triangulation principle is applied. If a pattern is projected onto the object surface, the distance information in respect of all points of the pattern in a camera image can be calculated.
U.S. Pat. No. 4,761,071 shows a device and a method for determining the topography of the cornea and the sclera of the eye by means of triangulation. Here, a fluorescence substance is introduced into the eye and illuminated by an incident light beam. The fluorescence radiation is captured from another direction and the elevation of the eye surface is determined by triangulation.
In most known eye tracking systems, the eye position is captured by means of recorded camera images in a plane which is usually situated in the eye plane which is parallel to the transversal/vertical body plane (x-y plane). However, in order to determine the eye position completely, establishing the tilt of the eyeball is of importance in addition to knowledge of the lateral position of the pupil (x-y position) and the rotation of the eye about the viewing direction (z-direction). If the tilt is disregarded, there is a decentration error DE on the eye surface.
The decentration error is shown in FIG. 2. Here, a pupil 2 of an eye and the cornea 4, situated thereover at a distance APC, is schematically illustrated in two positions, namely initially in a first state 2a without tilt and furthermore in a second state 2b, in which it is tilted toward the right in the image. The angle α describes the angle of the tilt with respect to the normal state in which the eye is not tilted. The pupil is situated at a distance PCR from the center of rotation 1 of the eye. In the non-tilted, first state 2a, a central point 4a of the cornea 4 is situated precisely above the center of the pupil 2. If an image of the eye is captured from the viewing direction, the point 4a on the cornea 4 is situated at the same point in the image as the pupil center. If the eye is tilted about the center of rotation 1, this results in a lateral displacement LS of the center of the pupil 2 in the captured image of the eye. However, the central point 4a of the cornea 4 in the captured image of the eye no longer lies at the same point as the pupil center but rather is situated at a distance therefrom, which forms the decentration error DE.
By way of example, if in the case of laser ablation the point 4a is tracked as a predetermined ablation center in accordance with the lateral displacement LS, this results in a current ablation center 3 on the cornea 4 which is displaced by the decentration error DE with respect to the predetermined ablation center 4a. This results in an ablation which deviates from the intended position despite tracking. If the decentration error DE is to be taken into account, the tilt of the eye needs to be known.
The knowledge of the tilt angle is a necessary but not sufficient condition for correcting the error DE. Thus, for an exact correction, further parameters need to be known: the distance APC between pupil 2 and cornea 4 and the distance PCR between the center of rotation 1 and the pupil 2. If statistical means are used for these parameters, the error DE is at least reduced. However, the missing parameters are usually known from preliminary examinations.
In the case of laser ablation, the importance of correcting the treatment decentration, which is created by the tilt of the eye, increases with increasing shot frequency. If a frequency of approximately 100 Hz is assumed, a typical treatment has a duration of about 100 s. If the tilt of the eye is normally distributed in terms of direction and magnitude at this time, this then leads to the treatment “smearing.” Although the amount of volume ablated is approximately as intended, the treatment diameter increases and so the achieved ablation depth decreases as a function of the standard deviation of the tilt movement. If the tilt during the treatment is not distributed normally or, in an extreme case, systematically tilted, the ablation for various directions will deviate to a different extent from the intended ablation even in the case of low shot frequencies, and induced refractive errors may occur. If the frequency is e.g. 1000 Hz, the duration of a typical treatment reduces to 10 s. The probability of the tilt of the eye being distributed normally during this short period of time reduces and short deviations from the viewing direction can have drastic effects.
In order to establish the tilt of the eye, document DE102004025999A1 shows a device which comprises a reference object which is positioned on the eye. A sensor apparatus captures the reference object in two translational directions, and three further sensors capture the respective distance from three scanning points on the reference object. A computer unit establishes the position of the reference object in real time. However, a disadvantage of this device is that the reference object in the form of a ring needs to be placed onto the eye such that it is concentric with the pupil.